Interest has grown in the use of laser activated remote phosphor (LARP) for technology in various lighting applications, such as automotive, projection, and other lighting applications. One reason for that interest is that LARP technology has the potential to enable to production of lighting devices that can generate significantly higher luminance than devices that utilize light emitting diodes (LEDs), at relatively high power levels.
FIG. 1 depicts one example of a LARP system. As shown, LARP system 100 includes a first light source 101 in the form of a laser. The first light source 101 emits rays 103 of laser light towards a dichroic beam splitter 105. The dichroic beam splitter 105 reflects rays 103 into a collimating optic 107. The reflected rays 103 pass through and are focused by the collimating optic 107 onto a wavelength converter 109 that is present on a substrate 111. The wavelength converter 109 includes a wavelength conversion material that functions to convert (e.g., via photoluminescence) at least a portion of light rays 103 incident thereon to light of a different wavelength than light rays 103, in this case light rays 115. As significant heat may be generated by the conversion of rays 103 to rays 115, a heat sink 113 may be coupled to the substrate 111 so as to facilitate the dissipation or removal of excess heat.
At least a portion of the rays 115 produced by wavelength converter 109 are collected by the collimating optic 107 and are redirected back through the dichroic beam splitter 105, where they are incident on a focusing lens 121. The focusing lens 121 focuses rays 115 on other components 123 of LARP system 100, such as fiber/projection optics.
LARP system 100 may also include an optional second light source 117 (e.g., a laser or non-laser source), as shown. When included the second light source 117 may be used to emit light rays 119 that reflect off of the dichroic beam splitter 105 towards the focusing lens 121. The resultant mixing of rays 119 and rays 115 may result in a corresponding change in the color temperature or other properties of the light in the region down field of the dichroic beam splitter 105.
Using such a configuration tens of watts of laser light (i.e., rays 103) may be pumped into a small (e.g. square-millimeter (mm2) area of wavelength converter 109, resulting in the production of broad or narrow-band emission of secondary light (i.e., rays 115) with a relatively low étendue and a relatively high light output (e.g., from several hundred to above 10,000 lumens). LARP systems such as the one shown in FIG. 1 may therefore considered attractive for many projection applications such as digital micro-mirror (DMD) modulators, fiber optic sources, and the generation of highly collimated beams.
While LARP systems have shown some promise, challenges exist that have limited their practical implementation in various lighting applications. One such challenge is that the wavelength converters used in many LARP systems often produce secondary light in a hemispherical (approximately Lambertian) pattern. For the system to be efficient, the collimating optic in the system needs to be able to capture a large fraction of the hemispherical luminescence produced by the wavelength converter. Capturing sufficient amounts of such light with traditional collimating optics can be difficult, and therefore special non-imaging type optics (e.g., a tapered total internal reflection optic as shown in FIG. 1) or very low F/number aspheric lenses (often more than one) are often used as collimating optics in LARP systems. Those specialized optics are often expensive, heavy, and can take up considerable space. It may also be necessary to place them very close to the surface of the wavelength conversion material (e.g., less than 100-200 microns (μm)) which can make alignment difficult.
Similar challenges exist with collimating optics used in optical applications outside of the context of a LARP system. For example in some LED projection systems, one or a plurality of non-laser, high luminance LEDs is/are used emit light into a hemisphere after which the emitted light is collimated by one or more collimating optics. One method of collimating the light emitted by an LED is to encapsulate the LED die in a lens. Although encapsulation can improve the light extraction efficiency of the LED, it may undesirably increase the étendue of the LED by a factor of n2, where n is the refractive index of the lens medium. An alternative approach may therefore be needed in instances where maintenance of étendue is desired, such as in light projection systems.
One such alternative approach is to use collimating optics similar to those used in the LARP system of FIG. 1 (either alone or in combination with an encapsulating lens if the increased étendue can be tolerated) to collimate light emitted by an LED. This concept is illustrated in FIG. 2, which depicts one example of a collimation system 200 in which a spatially extended light source 201 (e.g., an LED) emits rays 203 of light towards a collimating optic 205, with the light source 201 being aligned with the optical axis 207 of the collimating optic 205. In such instances, however, the same challenges associated with the collimating optics used in a LARP system (i.e., size, weight, alignment, cost, etc.) are presented.
An interest therefore remains in the development of alternative optics that are suitable for use in various applications such as LARP, high luminance LEDs, point source collimation, laser-based microscopy and other applications in which high numerical aperture collimation is desired. As will be discussed in detail below, the present disclosure generally relates to such alternative optics (and in particular metalenses) which are suitable for those and other applications.